Heretofore, a conventional starter device used for starting the engine of an automobile is of a so-called biaxial structure wherein an electromagnetic switch device for feeding power to a d.c. motor is disposed at the side of the d.c. motor.
However, such biaxial type starter device gave a great restriction when an engine for an automobile is to be designed.
There has been a proposal to make the shape of a starter device simple. According to the proposal, the starter device is so constructed that it has a generally elongated cylindrical form by arranging an electromagnetic switch device at an end in the axial direction of a d.c. motor.
The proposed starter device is illustrated in FIG. 3. In FIG. 3, a coaxial type starter device 1 is such that an armature rotary shaft 3 of a d.c. motor 2 is hollow; a plunger rod 4a of an electromagnetic switch device 4 disposed at the rear end of the d.c. motor 2 is extended in the inner passage 3a of the hollow armature rotary shaft 3; and an output rotary shaft 5 is inserted in the armature rotary shaft 3 in a coaxial manner so that the rear end of the output rotary shaft 5 is in contact with the free end of the plunger rod 4a, whereby when the electromagnetic switch device is actuated, the output rotary shaft 5 is pushed forwardly by means of the plunger rod 4a .
A sun gear wheel 6a is formed in the outer circumferential portion of the front end of the armature rotary shaft 3, and a plurality of planet gear wheels 6b are interlocked with the sun gear wheel 6a. The planet gear wheels 6b are also interlocked with an internal gear wheel 6c formed in the inner circumferential surface of a machine frame 7, and each of the planet gear wheels 6b is supported by a carrier 6e by means of each shaft 6d. The sun gear wheel 6a, the planet gear wheels 6b, the internal gear wheel 6c, the shafts 6d and the carrier 6e constitute a planet gear speed-reducing device 6 which reduces a rotating speed of the armature rotary shaft 3.
An overrunning clutch device 8 is fitted to the output rotary shaft 5. A clutch inner member 8a of the overrunning clutch device 8 is interlocked with a helical spline 5b formed in a spline forming section 5a of the output rotary shaft 5, the spline forming section 5a having an outer diameter greater than the inner diameter of the inner passage 3a of the hollow armature rotary shaft 3, whereby the output rotary shaft 5 can receive a rotating force from the clutch inner member 8a and is slidable in its axial direction. A pinion 9 is attached to the front end of the output rotary shaft 5 so as to be engaged with and disengaged from a ring gear of an engine (not shown). When the output rotary shaft 5 is caused to slide forwardly, the pinion 9 is engaged with the ring gear to rotate it.
However, the conventional coaxial starter device as described above had a disadvantage as follows. When the engine or the starter device is in an idling operation and if the key switch of the automobile is turned on, the pinion of the starter device comes to engagement with the ring gear of the engine to cause a shock to the structural elements of the starter device. In this case, the shock can hardly be absorbed by an absorbing energy by the torsional regidity of the overrunning clutch, and the shock sometimes breaks the structural elements.